Transactional email
- Triggered by one person's action: sign-up confirmations, OTPs, password resets, receipts, shipping updates, alerts.
- Expected and time-sensitive. People wait for them.
- Usually doesn't need an unsubscribe link, because it's needed to use your service.
Marketing email
- Sent to a list or segment: newsletters, offers, announcements, product updates.
- Needs consent and a clear one-click unsubscribe.
- More likely to get spam complaints, so it carries more reputation risk.
Why send them separately
If a newsletter gets complaints, inbox providers trust your sending less. When both kinds share one stream, your password resets suffer for your newsletter's sins. Separate streams (and ideally a subdomain like news.yourdomain.com for marketing) keep critical email flowing.
The grey area
A receipt with a big promotional banner starts to look like marketing. Keep transactional email focused on the transaction, and put promotions in your marketing email.